Subject [firebird-support] "could not convert variant of type (null) into type (currency)"
Author Rustam

If your client app written in Delphi, there is bad usage of Variant type in your code (or external components).

Absent one of checks when converting Variant to Currency, e.g.:

procedure TForm1.Button1Click(Sender: TObject);
   C: Currency;
   V: Variant;
   V := 10;
   C := V; // Works
   Edit1.Text := FloatToStr(C);

   V := Null;
   C := V; // Error
   Edit1.Text := FloatToStr(C);

   V := Null;
   // Correct usage
   if VarIsNull(V) then C := 0 else C := V;
   Edit1.Text := FloatToStr(C);